1. Field of the Invention
The invention is generally related to techniques which improve the bony fixation and support of a prosthesis. The invention has particular application in total hip replacement operations, wherein the technique utilizes an acetabular cup designed to resist loosening and migration.
2. Description of the Prior Art
Total hip replacement (THR) is one of the most beneficial surgical procedures when outcome is analyzed on a cost/quality-adjusted-life year basis (See, Sledge NIH consensus study September, 1994). The operation provides the patient with increased mobility and independence, thus improving overall quality of life. More than 800,000 artificial hips have been implanted in Americans so far. THR is one of the most commonly performed operations in orthopedic reconstructive surgery today. Over 20,000 hip replacements are performed annually in the United States at an annual nationwide cost in the billions. In 1994, the leading problem in THR is the failure of long term fixation of the acetabular component in the pelvis, and, specifically, the inability of the cup to resist loosening and migration. The 120,000 per year rate includes an increasing number of revision surgeries (second operations) due to failure. The total number of THR operations performed in the United States is expected to increase as the population ages and the currently implanted prostheses begin to fail.
FIG. 1 generally shows a hip replacement installed in a human patient. A stem 10 is anchored in the femur 12 and projects a ball (not shown) proximally into an acetabular component 14 that is positioned in the pelvis 16. Many prior art THRs utilize bone cement 13 for fixating the components to bone. Implanting an artificial hip requires surgical removal of the upper end of the femur, and reaming out a portion of the bone in the acetabulum (socket) of the pelvis. A new acetabular cup 14 is then installed in acetabulum and the stem 10 is inserted into the center of the femur 12. The stem 10 is typically a metallic component such as titanium or cobalt/chromium alloy, and the head (not shown) connected to the stem has a highly polished surface to enhance free rotating motion inside the acetabular cup 14. A plastic liner inside the acetabular cup 14, or construction of the entire acetabular cup 14 from plastic components can aid in permitting free motion of the patient's hip.
A common problem with hip replacements is related to loosening of the artificial socket and migration out of the socket into the pelvis. Loosening and migration of the socket results in loss of bone in the acetabulum. Bone loss can occur in the superior, posterior, anterior, or medial direction, all of which are commonly referred to as segmental defects. In addition, a volummetric loss in the bony substance in the pelvis can occur, and this is commonly referred to as a cavitary defect. Loss of bone in the acetabulum eventually requires replacement of the artificial hip because of pain and shortening of the leg. However, due to the loss of bone in the acetabulum, the revision surgery is likely to be less successful than the first surgery and invariably requires a bone graft.
There are many different configurations for both the stem 10 and the acetabular cup 14. For example, U.S. Pat No. 5,108,432 to Gustavson, U.S. Pat. No. 4,714,470 to Webb et al., U.S. Pat. No. 4,763,409 to Van Kampen, U.S. Pat. No. 4,778,469 to Lin et al., U.S. Pat. No. 4,535,487 to Esper et al., U.S. Pat. No. 4,863,474 to Brown et al., and U.S. Pat. No. 5,021,063 to Tager all disclose variations on the stem 10 section of a hip replacement, and U.S. Pat. No. 5,171,287 to Willert et al., U.S. Pat. No. 5,314,488 to Hayashi et al., and U.S. Pat. No. 5,314,490 to Wagner et al. all show variations on the acetabular socket. These patents reflect the commonly believed concept that the primary goal of the prosthesis-bone interface is fixation and adherence of one surface to another.